Distance measuring devices and in particular optoelectronic distance measuring devices have long been known per se. These devices transmit a modulated measurement beam, for instance a beam of light or a laser beam, which is aimed at a desired target object whose distance from the device is to be ascertained. The returning measurement signal reflected or scattered by the target object aimed at is detected again at least in part by a sensor in the device and used to ascertain the distance sought.
In the known devices of this type, a distinction is made between so-called phase measurement methods and pure transit time measurement methods for determining a sought distance from the target object. In the transit time measurement method, a pulse of light of the briefest possible pulse duration is transmitted by the measuring device, and then its transit time to the target object and back into the measuring device is ascertained. With the known value of the speed of light, the distance of the measuring device from the target object can be calculated from the transit time.
In the phase measurement method, conversely, the variation in the phase of the measurement signal is utilized as a function of the distance travelled to determine the distance between the measuring device and the target object. From the magnitude of the phase displacement impressed on the returning measurement signal, compared to the phase of the transmitted measurement signal, the distance travelled by the measurement signal can be determined, and thus the distance between the measuring device and the target object can also be determined.
The range of application of the distance measuring devices generally covers distances in the range from a few centimeters to several hundred meters. Meanwhile, such measuring devices are commercially sold in compact versions and make it simple for the commercial or private user to operate them, even in handheld form.
To attain high measurement precision with these devices, it is known to select and use as high as possible a modulation frequency. However, since nonambiguity of the phase measurement exists only for a phase angle between 0 and 360°, it is usual and also known for instance from German Patent Disclosure DE 43 03 804 A1 to alternate a high modulation frequency of the transmitted light beam with at least one further, substantially lower modulation frequency of the transmitted light beam, in order to attain a measurement range that goes beyond the phase angle range of 0 to 360° for the high modulation frequency.
It is also known, for more-precise ascertainment of a phase difference between the transmitted and the received measurement signal, to transform the signal to be analyzed to a markedly lower frequency, for instance by a frequency mixing process. This mixing process yields a low-frequency measurement signal which continues to be a carrier of the fundamental information, namely the phase displacement between the transmitted and the received signal, but because of its markedly reduced frequency is also substantially simpler to process further and can be evaluated more precisely.
To attain “downward mixing” of the measurement frequency, it is known to mix the transmission and reception signals with a signal whose frequency is displaced only far enough from the measurement frequency that an outcome of mixing is in the low-frequency range. In this low-frequency range, it is then no problem to measure the desired phase by means of a suitable switching device. Advantageously, the diode that detects the returning measurement signal can be used for this frequency mixing process.
From German Patent Disclosure DE 37 43 678 A1, an optical backscattering measuring device is known which has an optical transmitter whose transmission power can be modulated via an oscillator with a varying frequency. The transmission beam of the backscattering measuring device of DE 37 43 678 A1 is carried via a beam splitter into the optical waveguide to be examined. The portions of the transmitted beam that are backscattered by the optical waveguide are carried via the beam splitter to an optical receiver, embodied as a photodiode, of the optical backscattering measuring device. For ascertaining the location and intensity of the backscattering, in the backscattering measuring device of DE 37 43 678 A1, a mixed signal is formed from a signal that is proportional to the optical backscattering power and a modulation voltage that has the oscillator frequency. The expense for the optical receiver is reduced by providing that the photodiode is an avalanche photodiode, whose bias voltage is a direct voltage modulated by the modulation voltage. The low-frequency mixed signal thus generated is picked up at a parallel circuit, connected into the exciter circuit of the photodiode, that comprises an active resistor and a capacitor.
From European Patent Disclosure EP 0 932 835 B1, an apparatus for calibrating distance measuring instruments is known that has a transmitter which emits a high-frequency-modulated optical radiation and with it illuminates a measurement object. The apparatus of EP 0 932 835 B1 furthermore comprises a measurement receiver, which detects the radiation reflected by the measurement object and converts it into an electrical signal. From the transmitter beam path of the distance measuring instrument of EP 0 932 835 B1, some of the high-frequency-modulated transmitter radiation is permanently out-coupled and delivered, via an internal reference path serving as a calibration path, directly to a reference receiver, such as a PIN diode. This diode is connected to a frequency mixer. The frequency mixer is in turn connected directly to an avalanche photodiode, used as a measurement receiver for the measurement beam. A high-frequency electrical signal is coupled as a mixer frequency into this connection. This mixer frequency is mixed, via the frequency mixer, with the high-frequency modulation signal of the reference beam received from the reference receiver, and the result is a low-frequency calibration signal. On the other hand, the mixer frequency is mixed with the high-frequency modulation signal of the measurement beam received by the avalanche photodiode, and a result is a low-frequency measurement signal. Thus in the apparatus of EP 0 932 835 B1, the avalanche photodiode is a so-called direct mixer. The low-frequency calibration signal and the likewise low-frequency measurement signal are then delivered in a known manner to where the phase measurement is done.